1. Field of the Invention
This invention relates to a solenoid drive circuit in which the solenoid is quickly restored.
2. Background of the Invention
When it is required to deenergize a solenoid, a diode is connected in parallel with the solenoid. In the case where the cloth pressing plate (presser foot) of a sewing machine is moved up and down by a solenoid, the solenoid is energized to move the cloth pressing plate upwardly, and it is deenergized to move the cloth pressing plate downwardly.
For instance, in a sewing operation in which a label is sewn on a piece of cloth, the cloth pressing plate must be lifted whenever it is required to change the stitching direction. That is, it is necessary to frequently move the cloth pressing plate up and down. Accordingly, in order to increase the sewing efficiency and productivity, it is desirable that the time required for moving the cloth pressing plate up and down is as short as possible. The time required for moving the cloth pressing plate upwardly is determined by the characteristic of the solenoid used. On the other hand, the time required for moving the cloth pressing plate downwardly can be decreased by connected a resistor in series to the diode which is connected in parallel to the solenoid. Therefore, the resistor is generally employed.
In a typical sewing operation, the period of time during which the cloth pressing plate is held at the lower position is longer than the period of time for which it is held at the upper position. Therefore, in order to decrease the power consumption of the solenoid, the solenoid is energized when it is required to lift the cloth pressing plate. However, if the cloth pressing plate is held at the upper position for a long period of time, for instance because of a sewing pattern, the cloth pressing plate is kept energized for a long period of time. As a result, the solenoid generates a large quantity of heat. In order to minimize the heat generation of the solenoid, a method has been employed in which, as shown in the part (a) of FIG. 1, the solenoid is continuously energized until the cloth pressing plate has been lifted, and thereafter it is intermittently energized with a period short enough to maintain the cloth pressing plate at the upper position.
If the solenoid is energized intermittently as described above, current flows in the series circuit of the resistor and the diode which is connected in parallel with the solenoid whenever the energization of the solenoid is turned off, and therefore the resistor generates a large quantity of heat. Accordingly, the resistor must be large in power capacity, and accordingly large in size. Therefore, the resistor presents little freedom in where it can be installed, and it is not economical.